1. Field of the Invention
The present invention relates to the preparation of microcrystalline cellulose products (crystallinity of at least 78%) via enzymatic hydrolysis of certain cellulosic materials. The invention further relates to the enzymatic production of said microcrystalline cellulose exhibiting high crystallinity at LOPD.
2. State of the Art
Crystalline cellulose, or MCC, defined as cellulose with a crystallinity of at least 78%, has a wide variety of applications such as tabletting excipient, anti-caking aids, filler, carrier, formation of stable dispersions (alone in co-processed forms with other materials such as CMC, whey protein and so on), source of fiber, etc., for the food, pharmacy, and cosmetic industries.
The current commercially available forms of microcrystalline cellulose (MCC) are produced from a high grade of dissolving pulps and hydrolyzed to LODP of about 200-300 and contain crystallinities as measured by x-ray diffraction of greater than 78%. They are produced, as described by U.S. Pat. No. 2,978,446 to Battista, et al., by subjecting wood pulp containing amorphous and crystalline forms of cellulose to acid hydrolysis at elevated temperature, usually in 2.5 normal hydrochloric acid, on the order of 105.degree. C. or greater, for about 15 minutes to one hour, followed by mechanical disintegration of the cellulose hydrolysate. The process is generally carried out by hydrolyzing a pure grade of wood pulp with hydrochloric acid in order to prevent the formation of undesirable by-products from secondary reactions. The hydrolysis reaction removes amorphous cellulose and reduces the degree of polymerization of the cellulose chain leaving a substantially insoluble material, which is commonly referred to in the art as "level-off degree of polymerization" (LODP) cellulose.
The LODP value is dependent primarily upon the starting cellulosic material and to a lesser extent upon the severity of the hydrolyzing conditions. In general, the LODP of native cellulose fibers is in the range of between about 200-400, whereas that derived from regenerated cellulose lies in the range of from 25 to about 60.
It has been the general view of those knowledgeable about the manufacture and applications of microcrystalline cellulose powder that the only way to achieve the LODP value for a given cellulosic material, with crystallinity above about 78%, was by acid hydrolysis, as described above. This view was based primarily on the belief that only inorganic acids had the requisite reactivity and accessibility to attack the more tightly bound portions of the cellulose structure, and produce LODP cellulose at high crystallinity of 78% or more. It was previously thought that microcrystalline cellulose could not be produced at other than the LODP of the starting material.
In the prior art, although other methods of cellulose degradation, e.g., enzymatic hydrolysis, have been studied and reported such methods have not been considered appropriate for commercial microcrystalline cellulose powder production with LODP because they have not produced crystallinity above 78%.
In U.S. Pat. No. 4,427,778 (Zabriskie), a method using cellulase for making cellulose powder suitable for tabletting was described using a pH of about 4-5 at temperatures around 50.degree. C. DP's were obtained down to about 880 or so, and the percent crystallinity reported never got above about 75%, i.e., never made microcrystalline cellulose and never achieved LODP.
Some difficulties with the acid hydrolysis method are the energy required to boil the acid, the corrosive nature of the acid, the inability to control the reaction and the inability to obtain DP's other than LODP. In addition, a problem with the previous enzymatic method is that crystallinity greater than 75% is never achieved and DP's below about 80 are not seen.
The present invention provides an enzymatic route to producing LODP MCC enzymatical for use as an alternative in the manufacture of various articles.
It is also now possible to produce MCC at greater than 78% crystallinity and greater than LODP.